There are a variety of known processes for producing hydrogen. Some examples include the following: (1) steam reforming of natural gas or naphtha, (2) catalytic reforming of hydrocarbons, e.g. gasoline and fuel oil, and (3) partial oxidation of heavy oils or natural gas. In the aforementioned processes, steam reforming of natural gas is probably the most widely used process for hydrogen production. FIG. 1 shows the key process units in a steam methane reforming process to produce hydrogen. Referring to FIG. 1, a feedstock, e.g., natural gas is compressed and fed to a purification unit to remove sulfur compounds. The desulfurized feed is then mixed with superheated steam and fed to a reformer to produce primarily H.sub.2 and CO. The effluent stream from the reformer is sent to a heat recovery unit, then to a shift converter to obtain additional hydrogen. The effluent from the shift converter goes through a process cooling and recovery unit prior to sending to a purification unit (e.g. PSA) to produce high purity hydrogen. The following gives a brief introduction of some of the prior art processes for producing hydrogen.
Sircar et al., in U.S. Pat. No. 4,077,779, describe an improved four bed hydrogen PSA process; wherein, a cocurrent displacement step, commonly referred to as high pressure rinse step, is used prior to depressurization of each adsorbent bed. By using a high pressure rinse step prior to depressurization of the bed, enhanced light component recovery is achieved. In addition, according to this invention, an evacuation step is used between the purging and repressurization steps, and high purity H.sub.2 (99-99%) can be produced at high recovery (96.5%) from a feed of 75% H.sub.2 and 25% CO.sub.2.
Fuderer et al., in U.S. Pat. No. 5,553,981, disclose a process for enhancing hydrogen recovery by combining shift conversion, scrubbing and PSA. Carbon dioxide is mainly removed by scrubbing. Hydrogen is then purified by PSA. The recycling of a portion of the waste from PSA system to shift conversion unit improves the hydrogen recovery. In an example case, H.sub.2 can be finally purified by PSA up to 99.99% from a feed typically around 97% after methanation. The recovery can reach 99% if 80% of PSA waste is recycled to the shift converter, 15% is recycled to the partial oxidation unit and 5% is discharged for use as fuel gas.
In U.S. Pat. No. 5,152,975, Fong et al., disclosed a process for producing high purity hydrogen. The basic steps in the process include the following: (1) partially oxidizing a gaseous hydrocarboneous feedstock to produce a synthesis gas mixture of H.sub.2 and CO, (2) reacting the synthesis gas mixture with steam to convert CO into a raw gas mixture that primarily contains CO.sub.2 and H.sub.2, and (3) passing the raw gas mixture to a PSA process to produce high purity hydrogen and a reject gas mixture of impurities.
Kapoor et al., in U.S. Pat. No. 5,538,706, disclose a process for hydrogen and carbon monoxide production from hydrocarbons. It consists of partial oxidation and PSA separation, and cryogenic distillation as one possibility. The system provides a better performance by improving PSA separation and reducing starting materials. The partial oxidation reactor produces high purity hydrogen and carbon monoxide. Hydrogen and carbon monoxide are separated from the gas mixture (containing carbon dioxide, methane and water vapor, etc.) by a combination of PSA and distillation or possibly only PSA. At least one stream rich in high hydrocarbons (and water vapor etc.) is recycled to the reactor. A preferred embodiment of PSA system comprises serially connected adsorption zones (or beds of zeolite 5A or 13X), in each zone carbon monoxide is less strongly adsorbed than higher hydrocarbons, carbon dioxide and water vapor, but more strongly than hydrogen. According to this invention, a basic PSA cycle is used, and the depressurization/desorption step is divided into two parts: the first part, middle depressurization during which the top bed undergoes countercurrent depressurization while the bottom bed undergoes cocurrent depressurization, and carbon monoxide is withdrawn between two adsorption beds; the second part is a normal countercurrent depressurization for both beds. The process can produce, as claimed, at least 98% pure hydrogen and carbon monoxide, and the gas entering the PSA unit has about 55% H.sub.2 and 42% CO.
McCombs et al., in U.S. Pat. No. 4,263,018, disclose a pressure swing adsorption process utilizing at least two adsorption beds, and a storage tank that is either an empty tank or a packed bed. The storage tank is used for storing equalization falling gas from the top of the bed that is subsequently used to pressurize another bed during the equalization rising step. This invention practices a sequential refluxing strategy wherein the void gas captured during the equalization falling step is returned in the order of increasing purity during the equalization rising step. In addition, this invention discloses the use of bottom-to-bottom bed equalization step; wherein, the bed receiving the bottom equalization rising gas is also receiving feed simultaneously, i.e., uninterrupted feed during the bottom-to-bottom bed equalization step.
Yamaguchi et al., in U.S. Pat. No. 5,258,059, describe a PSA process using at least three adsorbent beds and a holding column (segregated storage tank) of the feed-in/feed-out sequence retaining type. The holding column is used for storing the void gas recovered during the cocurrent depressurization step of the cycle. This gas is then used for purging of the adsorbent bed during the regeneration step of the cycle. This holding column is specifically designed to prevent gas from mixing, i.e., an impurity concentration gradient exists in this holding column. This invention is restricted to the use of at least three adsorbent beds and a holding column to store cocurrent depressurization gas that is subsequently used for purging. It does not describe for example, how this holding column can supply both the purge gas and product pressurization gas in advanced PSA cycles.
Baksh et al. in U.S. Pat. No. 5,565,018, disclose the use of segregated external gas storage tanks to store gases of varying purity for later use, in the order of increasing purity, in the purging, equalization rising, and product pressurization steps of the PSA cycle. Significant reduction in both the bed size factor and power consumption are achieved with this novel PSA cycle.
Although, many modifications and variations of the basic PSA cycle have been studied and applied to commercial processes, such as for the production of hydrogen from synthesis gas, yet PSA processes remain inefficient and uneconomical for high purity production of hydrogen for large plants when compared to the alternative methods. In particular, current PSA processes (such as those disclosed above) both produce an enriched product having a purity that is averaged over the whole "make product" step and possess several inherent problems.
For example, during the top-to-top bed equalization step, the bed going through the equalization rising step receives product gas with decreasing purity. Consequently, at the end of this equalization rising step, the lowest purity gas is at the product end of the bed. Thus, during the subsequent make product step, the product purity spikes downwards, which when averaged over the whole production step, decreases the purity of the product. In addition, the gas used for purging is of decreasing purity when obtained from another bed currently in the make product step. If this purge gas was obtained from a storage tank, then a constant purity gas is used for purging. Also, a similar problem (using products of decreasing purity) exists in PSA cycles that utilized a product pressurization step in the PSA cycle, wherein, the product gas was obtained from another bed in the production step.
Typically, in prior art H.sub.2 PSA cycles, the bed going through the equalization rising step receives product gas with decreasing purity. Consequently, at the end of this equalization rising step, the lowest purity gas is at the product end of the bed. In addition, the gas used for purging of the adsorbent bed was obtained during a second stage equalization falling step, and is used in the order of decreasing purity. The statements made for the equalization and purging steps are equally valid for the gas used in PSA cycles that utilized a product pressurization step. In addition, during the bottom-to-bottom bed equalization step, the bed that is being pressurized is receiving gas of decreasing hydrogen concentration, starting with a H.sub.2 concentration that is equal to the feed concentration, and as the bottom-to-bottom bed equalization step continues, the hydrogen concentration at the feed end of the bed drops due to heavy component(s) desorption.
The aforementioned concentration degradation, i.e., gas of decreasing H.sub.2 concentration, is being used for refluxing and repressurization, giving rise to process irreversibility and mixing losses, viz, a degradation in the PSA process efficiency. For example, during the purge step and top-to-top bed equalization step, gas of decreasing H.sub.2 concentration is being used for refluxing. During the bottom-to-bottom bed equalization step, at the start of the equalization, the H.sub.2 mole fraction is about 0.7403 (same as the feed concentration in synthesis gas), and as the bottom-to-bottom equalization progresses, the hydrogen mole fraction at the feed end of the bed falls from about 0.74 to about 0.65. Thus, in order to maintain desired product purity in prior art PSA cycles, the production and equalization falling steps must be terminated much earlier than the time required for the bed to breakthrough. This results in a failure to fully utilize the adsorbent bed.
Further, by using products of decreasing purities, for example during purging and repressurization steps, additional contamination of the product end of the bed results, due to the use of the lowest purity product gas at the end of the refluxing steps. This added contamination of the product end of the bed brings about a significant reduction in the product purity in the early stage of the make product step. This causes a decrease of the average purity of the product, or for a given purity, a significant reduction in product (light component) recovery. In addition, by using product gas of decreasing purity, the spreading of the mass transfer zone is enhanced. Finally, in order to contain the mass transfer zone and maintain product purity, more adsorbent is required, resulting in higher bed size factor and a more costly PSA process.